1. Field of the Invention
The subject of the present invention is a rocket engine nozzle, exhibiting a jet separation control system, for example a device for injecting fluid through a wall of the nozzle, so as to induce jet separation in the gases ejected by the nozzle.
An important point in the design of a launcher is the optimization of the performance of its engines. In particular, the nozzle must be designed so as to yield a maximum thrust coefficient compatible with the limits imposed by the other constraints.
The thrust coefficient C of a nozzle is an increasing function of the ratio of the exit area Ae of the nozzle to the area At of the throat of the nozzle.
For an upper stage, which is ignited outside the atmosphere, the static pressure of the jet at the exit of the nozzle may be very low. The area ratio R=Ae/At of the nozzle is in this case essentially limited by the space available.
On the other hand, when the nozzle operates within the atmosphere, the gases which exit the nozzle cannot expand to below a limit pressure Psep, at which a separation of flow in the nozzle occurs spontaneously.
This jet separation is naturally unstable and generates considerable aerodynamic forces at the moment of ignition and during the initial atmospheric flight, which may even lead to the destruction of the nozzle if the jet separation is too considerable.
As far as engines which are designed to operate right from the ground and to accomplish the major part of their mission outside the atmosphere are concerned, the determination of the ratio Ae/At represents a difficult compromise.
Numerous devices have been proposed for controlling jet separation in nozzles.
An up-to-date review of this topic has appeared in particular in the article entitled Advanced Rocket Nozzles by Gerald Hagemann et al., published in the Journal of Propulsion and Power, vol. 14 No. 5, September–October 1998, pages 620 to 634.
2. Description of the Related Art
This deals in particular with “dual-bell” nozzles, nozzles with fixed or temporary inserts, two-position or extendible nozzles, external expansion nozzles, so-called expansion/deflection nozzles, nozzles exhibiting a variable throat area, and finally dual-mode nozzles.
The control of jet separation in a nozzle with the aid of secondary injection of gas has also been proposed, but this secondary injection has the effect of preserving axial symmetry of the flow. This technique is recalled in point 4, page 626 of the aforesaid article.
Experiments carried out on an RL10 engine and implementing passive injection are described in the article entitled “Altitude Compensating Nozzle Evaluation” by R. C. PARSLEY et al., published in the proceedings of the 28th Joint Propulsion Conference and Exhibit, 6 to 8 Jul. 1992, Nashville, Tenn., pages 1 to 6.
Finally, American patent U.S. Pat. No. 3,925,982 (Martin Marietta Corporation) describes a rocket engine exhibiting a high nozzle area ratio and which is equipped with a device for active secondary injection exhibiting a shock generating ring which is intended to control jet separation, by forcing the boundary layer of the primary gas jet to separate uniformly from the wall of the nozzle.
This is achieved with the aid of a large number of injection points which are distributed around the circumference of the nozzle. These injection points are closely spaced, and they inject a secondary gas jet radially and inwardly of the nozzle so as to effect jet separation which is invariant with any rotation about the axis of the nozzle.
Alternatively, this jet separation can be achieved via a continuous slot extending over the entire circumference of the nozzle.
The theory of jet separation has been recalled in the recent article by G. L. ROMINE entitled “Nozzle Flow Separation” published in the AIAA Journal, vol. 36, No. 9, September 1998, pp. 1618 –1625.
The theory of secondary injection has been set out in the article entitled “Some aspects of gaseous secondary injection with application to thrust vector control” by R. D. GUHSE et al., published in proceedings No. 71–750 of the AIAA/SAE 7th Propulsion Joint Specialist Conference of Salt Lake City, 14–18 June 1971, pages 1 to 8.
The known techniques of secondary injection, which involve jet separation exhibiting axial symmetry, that is to say which is invariant about any rotation about the axis of the nozzle, exhibit the following problems:                active secondary injection is difficult to implement, given that the mass flux which is required for effective generation of axial symmetric jet separation is high;        passive secondary injection which implements ventilation of the nozzle is operational only within a limited range of differential pressure, which implies that in order to obtain a nozzle which operates at all altitudes, its porosity must be continuously variable as a function of external pressure and of the operational parameters of the engine, this being hardly compatible with the nozzle construction constraints.        
One of the drawbacks of secondary injections with axial symmetry, such as for example that described in the aforesaid American patent, is that under certain engine operating conditions, the jet separation commences at a random point on the injection ring, and whose position, which depends on the upstream disturbances, is unstable.